To control the Bind of the Surface Deform Modifier with Python
Set the object active (and preferably, use the new modifier name instead of a constant name).
import bpy

obs             = bpy.context.selected_objects
modifier_type   = 'SURFACE_DEFORM'
modifier_target = bpy.data.objects['Plane']

for ob in obs :
    # Set the object active
    bpy.context.view_layer.objects.active = ob
    modifier = ob.modifiers.new(name=modifier_type,type=modifier_type)
    modifier.target = modifier_target
    bpy.ops.object.surfacedeform_bind(modifier=modifier.name)